In recent years, portable electronic appliances such as a notebook-sized personal computer, a portable telephone and a personal digital assistant have spread. Among secondary batteries used as a power source of the portable electronic appliances, lithium ion secondary batteries are widely used. To enhance the expediency of portable electronic appliances, they have been rapidly rendered small in size, thin in thickness, light in weight and high in performances. Consequently the portable electronic appliances have spread far and wide. With the spread thereof, requirements for rendering secondary batteries small in size, thin in thickness, light in weight and high in performances are becoming severe.
To meet the above-mentioned requirements, electrodes, electrolytes and other battery elements or parts are being examined. As for electrodes, an active material, a collector and a polymer binder for adhering an active material to a collector are being examined. Usually, a polymer binder is mixed with water or an organic liquid to obtain a binder composition, and the binder composition is mixed with an active material and optional electrically conductive carbon and other ingredients to give a slurry. A collector is coated with the slurry and then the liquid coating is dried to give an electrode.
As an example of the polymer binder, a polymer binder containing butadiene units has been proposed. For example, a butadiene rubber binder is described in Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) H4-255,670 and JP-A H7-335,221, and a polymer binder comprised of a styrene-butadiene latex is described in JP-A H9-320,604. A combination of a fluoropolymer with a polymer containing butadiene units is described in JP-A H6-215,761 and JP-A H9-213,337.
However, these heretofore proposed polymer binders containing butadiene units have been proved not to meet the requirements recently desired for achieving high battery performance such as satisfactory characteristics at repetition of charge-discharge cycles at a high temperature of 50° C. or higher or at a low temperature of 0° C. or lower, and satisfactory storage stability characteristics under temperature-varied conditions encountered when a high temperature storage and a low temperature storage are alternately repeated at heat-shock test (the storage stability characteristics are hereinafter referred to as “storage stability characteristics” for brevity when appropriate).